The present invention relates to a break-in method in a key system and, in particular, to a break-in method for switching a key system to a conference-call state by use of a conference-call circuit. In a break-in operation, when an attendant perceives a telephone subscriber on the line, the attendant breaks forcefully into the subscriber's call currently being made, and changes the current the present call to a conference call. Thereafter, if the attendant hangs-up on the phone, the subscriber having waited for being switched breaks into the conference-call replacing the attendant.
In general, a break-in system is used when an attendant wants to make an emergency call to an extension-line subscriber or to connect an urgent office-line call to the extension-line subscriber, and during the operation the present call must be continuously sustained.
However, in a conventional key system, when an attendant breaks into a extension-line subscriber's call, the attendant can only connect the lines, whereas the inventive method allows the subscriber, waiting for being switched, or a reserved subscriber, to break in the conference-call state by use of a conference-call circuit while continuously holding the subscriber's present line.